This invention relates to a photometering method in a photographic printer which is carried out by adjusting color balance and density by means of a color compensation filter, and irradiating a light on an original film through a mirror box for photoprinting.
As shown in FIG. 1, a photographic printer including a color compensation filter is so arranged that a film negative 2 on a film negative carrier 9 which is conveyed to a printing section is subjected to light from a light source 4 by the color compensation filter 3 consisting of three complementary color filters for yellow (Y), magenta (M) and cyan (C) and by a mirror box 1 whereas a transmitted light from the film negative 2 passes through a lens unit 5 and a black shutter 6 to a photographic paper 7. The photographic paper 7 is wound on a supply reel 7A and taken up on a take up reel 7B with simultaneous conveyance and stoppage thereof. Photosensors 8, such as photo diodes or the like for detecting image density information on the three primary colors such as red, green and blue are located adjacent to the side of the lens unit 5 for the film negative 2. The photographic printer as disclosed in Japanese Laid-open patent application No. 154244/1985 includes an image information detecting apparatus 10 composed of a two-dimensional image sensor 11 tilted toward an optical axis LS extending from the light source 4 and the film negative 2. This image information detecting apparatus 10 carries a lens unit 12 for forming the image with the approximate center of the film negative 2 as the optical axis. The detector is provided at its rear with a base plate 13 for mounting a processing circuit consisting of ICs and the like for the purpose of image processing. Normally, the photosensors 8 or the image information detecting apparatus 10 is adapted to detect image information on the film negative 2 to effect photoprinting by utilizing the optimum exposure amount suitable for the film negative in question.
However, a photographic printer of this class must have its mirror box 1 changed in accordance with the size of the film negative 2 being used and this means that the light quantity per unit area of the film negative 2 varies greatly. This in turn results in a considerable variation in the light quantity of an elementary image unit which is focused to form an image on the photometering image sensor 11 or which is condensed into the photosensors 8. Specifically, the image sensor normally covers a dynamic range which is too small to be practical, say about 1:100, so that the output amounts to a noise level (dark current) or a saturated level reverse thereto when the light quantity which is to be a reference for image information detection is noticeably changed, thereby to causing a difficulty in obtaining accurate detection of image information. In order to perform an effective printing operation by setting the printing and exposure time within a predetermined range even if the size of the film negative is varied, it is necessary to remove the mirror box 1 for the light source 4 and exchange it for another in accordance with the size of the film negative.
Generally speaking, the size combinations for the film negative and the mirror box 1 are as follows: EQU 120 size.fwdarw.120 mirror box EQU 135F, 135H, 126 size.fwdarw.135/126 mirror box EQU 110 size, disc film.fwdarw.110/disc mirror box
Now the, difference in light quantity per unit area of the film negative between a 120 mirror box and a 110/disc mirror box is more than 10-fold. For this reason, the dynamic range is decreased and there is difficulty in obtaining accurate image information detection when light is measured by the image sensor. This should be overcome at all costs.
Proposals for a remedy of this problem have been made wherein an auto iris or an ND (neutral density) filter adjusting mechanism is mounted in an image forming optical system for an image sensor. This provides a disadvantage, however, in that some difficulty is involved in adjustment of the sensitivity of each of the colors such as red, green and blue and in that the auto iris is expensive and complicated in structure and the ND filter adjusting mechanism fractionates light too continuously for it to be adjusted.